Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller-cone earth-boring rotary drill bits include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which the cone is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond compact cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high pressure and high temperature, typically in the presence of a metal solvent catalyst (such as cobalt, iron, nickel, or alloys or mixtures thereof) to form a layer or “table” of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high-pressure, high-temperature (of “HPHT”) processes. The metal solvent catalyst material may be partially dispersed within and between the compacted diamond grains prior to HPHT sintering or during sintering processes to promote diamond-to-diamond bonding, and to harden and strengthen the compacted diamond powder table.
The cutting element substrate may include a cermet material (i.e., a ceramic-metal composite material) such as cobalt-cemented tungsten carbide. In such instances, the metal solvent catalyst (e.g., cobalt, or other catalyst material) in the cutting element substrate may be drawn or imbibed into the compacted diamond grains or crystals during sintering and promote formation of a diamond table by binding the diamond grains or crystals together. In other methods, the metal solvent catalyst material may be mixed with the diamond grains or crystals as a powder, prior to sintering the grains or crystals together in an HPHT process. The metal solvent catalyst may be added to a compacted diamond feed as a powder and may be mixed with the diamond powders prior to the HPHT process.
Additional catalyst materials may be present in the metal solvent catalyst. For example, additional catalyst materials may be drawn from the substrate with the metal solvent catalyst during HPHT processes or may be added to a diamond compact powder as a foil, powder, or transient compound prior to sintering. The additional catalyst material may promote liquid phase sintering of the diamond by wetting and dispersion of the metal solvent catalyst around and through the compacted diamond grains. Such additional catalysts are often referred to as sweep catalysts and may improve sintering of superhard materials under HPHT conditions.
Upon formation of a diamond table using the HPHT process, the metal solvent catalyst material may remain in interstitial spaces between the grains of diamond in the resulting polycrystalline diamond table. The presence of the metal solvent catalyst material in the diamond table may contribute to thermal damage therein when the cutting element is heated during use, due to friction at the contact point between the cutting element and the formation.
Polycrystalline diamond compact cutting elements in which the metal solvent catalyst material remains in the diamond table are generally thermally stable up to a temperature of about seven hundred fifty degrees Celsius (750° C.), although internal stress within the cutting element may begin to develop at temperatures exceeding about four hundred degrees Celsius (400° C.) due to a phase change that occurs in cobalt at that temperature (a change from the “beta” phase to the “alpha” phase). Also beginning at about four hundred degrees Celsius (400° C.), there is an internal stress component that arises due to differences in the thermal expansion of the diamond grains and the catalyst metal at the grain boundaries. This difference in thermal expansion may result in relatively large tensile stresses at the interface between the diamond grains, and contributes to thermal degradation of the microstructure when polycrystalline diamond compact cutting elements are used in service. Differences in the thermal expansion between the diamond table and the cutting element substrate to which it is bonded further exacerbate the stresses in the polycrystalline diamond compact. This differential in thermal expansion may result in relatively large compressive and/or tensile stresses at the interface between the diamond table and the substrate that eventually lead to the deterioration of the diamond table, cause the diamond table to delaminate from the substrate, or result in the general ineffectiveness of the cutting element.
Furthermore, at temperatures at or above about seven hundred fifty degrees Celsius (750° C.), some of the diamond crystals within the diamond table may react with the metal solvent catalyst material causing the diamond crystals to undergo a chemical breakdown or conversion to another allotrope of carbon. For example, the diamond crystals may graphitize or form amorphous or glassy carbon at the diamond crystal boundaries, which may substantially weaken the diamond table. Also, at extremely high temperatures, in addition to graphite and amorphous or glassy carbon, some of the diamond crystals may be converted to carbon monoxide and carbon dioxide.
In order to reduce the problems associated with differences in thermal expansion and chemical breakdown of the diamond crystals in polycrystalline diamond cutting elements, so called “thermally stable” polycrystalline diamond compacts (which are also known as thermally stable products, or “TSPs”) have been developed. Such a thermally stable polycrystalline diamond compact may be formed by leaching the metal solvent catalyst material (e.g., cobalt) out from interstitial spaces between the inter-bonded diamond crystals in the diamond table using, for example, an acid or combination of acids (e.g., aqua regia). A substantial amount of the metal solvent catalyst material may be removed from the diamond table, or metal solvent catalyst material may be removed from only a portion thereof. Thermally stable polycrystalline diamond compacts in which substantially all metal solvent catalyst material has been leached out from the diamond table have been reported to be thermally stable up to temperatures of about twelve hundred degrees Celsius (1,200° C.). It has also been reported, however, that such fully leached diamond tables are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are non-leached diamond tables. In addition, it is difficult to secure a completely leached diamond table to a supporting substrate. In an effort to provide cutting elements having diamond tables that are more thermally stable relative to non-leached diamond tables, but that are also relatively less brittle and vulnerable to shear, compressive, and tensile stresses relative to fully leached diamond tables, cutting elements have been provided that include a diamond table in which the metal solvent catalyst material has been leached from a portion or portions of the diamond table. For example, it is known to leach metal solvent catalyst material from the cutting face, from the side of the diamond table, or both, to a desired depth within the diamond table, but without leaching all of the metal solvent catalyst material out from the diamond table.